Major Depressive Disorder (MDD) affects approximately 16% of Americans and represents one of the most common causes of disability worldwide. Unfortunately, less than one third of patients experience sustained remission of MDD symptoms with currently available medications. Recent studies have raised the exciting possibility that highly selective antagonists or negative allosteric modulators (NAMs) of the metabotropic glutamate receptor mGlu5 subtype may provide a novel approach to the treatment of MDD. This mechanism represents a fundamental departure from existing therapies and has the potential to provide more rapid onset than existing therapies and may provide sustained relief for refractory patients. In this revised NCDDG application, we propose studies aimed at using our novel mGlu5 NAMs discovered at the Vanderbilt Center for Neuroscience Drug Discovery (VCNDD) to further validate this mechanism in animal models and perform translational studies that can support a future clinical development effort. In addition, we will optimize novel mGlu5 NAMs to achieve properties required to select a lead clinical development candidate and backup compounds. This will require coordinated efforts of two major projects and support of 2 cores. Project 1 will focus on animal studies aimed at further characterizing the behavioral effects and CNS occupancy of novel mGlu5 NAMs. In Project 2, we will chemically optimize novel mGlf5 NAMs based on this scaffold to achieve a balance of properties required to select a clinical development candidate that can advance to IND-enabling studies and clinical development. These projects will be supported by two cores, including a Bioanalytical and DMPK Core (Core A) and Administrative Core (Core B).